Process for the production of acetic acid

ABSTRACT

A process for the reduction and/or removal of permanganate reducing compounds formed by the carbonylation of methanol in the presence of a Group VIII metal carbonylation catalyst to produce acetic acid is disclosed. More specifically, a process for reducing and/or removing permanganate reducing compounds or their precursors from intermediate streams during the formation of acetic acid by said carbonylation processes is disclosed. In particular, a process in which a low boiling overhead vapor stream from a light ends column is subjected to a single distillation to obtain an overhead that is subjected to an extraction to selectively remove and/or reduce PRC&#39;s from the process is disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for the production of acetic acidand, in particular, to an improved process for the reduction and/orremoval of permanganate reducing compounds formed by the carbonylationof methanol in the presence of a Group VIII metal carbonylation catalystto produce acetic acid. More specifically, this invention relates to animproved process for reducing and/or removing permanganate reducingcompounds or their precursors from intermediate streams during theformation of acetic acid by said carbonylation processes.

2. Technical Background

Among currently employed processes for synthesizing acetic acid, one ofthe most useful commercially is the catalyzed carbonylation of methanolwith carbon monoxide as taught in U.S. Pat. No. 3,769,329, issued toPaulik et al. on Oct. 30, 1973. The carbonylation catalyst containsrhodium, either dissolved or otherwise dispersed in a liquid reactionmedium or supported on an inert solid, along with a halogen-containingcatalyst promoter as exemplified by methyl iodide. The rhodium can beintroduced into the reaction system in any of many forms. Likewise, thenature of the halide promoter is not generally critical. The patenteesdisclose a very large number of suitable promoters, most of which areorganic iodides. Most typically and usefully, the reaction is conductedby continuously bubbling carbon monoxide gas through a liquid reactionmedium in which the catalyst is dissolved.

An improvement in the prior art process for the carbonylation of analcohol to produce the carboxylic acid having one carbon atom more thanthe alcohol in the presence of a rhodium catalyst is disclosed incommonly assigned U.S. Pat. Nos. 5,001,259, issued Mar. 19, 1991;5,026,908, issued Jun. 25, 1991; and 5,144,068, issued Sep. 1, 1992; andEuropean Patent No. EP 0 161 874 B2, published Jul. 1, 1992. Asdisclosed therein, acetic acid is produced from methanol in a reactionmedium containing methyl acetate, methyl halide, especially methyliodide, and rhodium present in a catalytically effective concentration.These patents disclose that catalyst stability and the productivity ofthe carbonylation reactor can be maintained at surprisingly high levels,even at very low water concentrations, i.e., 4 weight percent or less,in the reaction medium (despite the general industrial practice ofmaintaining approximately 14-15 wt % water) by maintaining in thereaction medium, along with a catalytically effective amount of rhodiumand at least a finite concentration of water, a specified concentrationof iodide ions over and above the iodide ion that is present as hydrogeniodide. This iodide ion is a simple salt, with lithium iodide beingpreferred. The patents teach that the concentration of methyl acetateand iodide salts are significant parameters in affecting the rate ofcarbonylation of methanol to produce acetic acid, especially at lowreactor water concentrations. By using relatively high concentrations ofthe methyl acetate and iodide salt, one obtains a surprising degree ofcatalyst stability and reactor productivity even when the liquidreaction medium contains water in concentrations as low as about 0.1 wt%, so low that it can broadly be defined simply as “a finiteconcentration” of water. Furthermore, the reaction medium employedimproves the stability of the rhodium catalyst, i.e., resistance tocatalyst precipitation, especially during the product recovery steps ofthe process. In these steps, distillation for the purpose of recoveringthe acetic acid product tends to remove from the catalyst the carbonmonoxide, which in the environment maintained in the reaction vessel, isa ligand with stabilizing effect on the rhodium. U.S. Pat. Nos.5,001,259, 5,026,908 and 5,144,068 are herein incorporated by reference.

It has been found that although a low water carbonylation process forproducing acetic acid reduces such by-products as carbon dioxide,hydrogen, and propionic acid, the amount of other impurities, presentgenerally in trace amounts, can be increased by a low watercarbonylation process, and the quality of acetic acid sometimes sufferswhen attempts are made to increase the production rate by improvingcatalysts, or modifying reaction conditions.

These trace impurities affect quality of acetic acid, especially whenthey are recirculated through the reaction process, which, among otherthings, can result in the build up over time of these impurities. Theimpurities that decrease the permanganate time of the acetic acid, aquality test commonly used in the acetic acid industry, include carbonylcompounds and unsaturated carbonyl compounds. As used herein, the phrase“carbonyl” is intended to mean compounds that contain aldehyde or ketonefunctional groups, which compounds may or may not possess unsaturation.See Catalysis of Organic Reaction, 75, 369-380 (1998), for furtherdiscussion on impurities in a carbonylation process.

The present invention is directed to reducing and/or removingpermanganate reducing compounds (PRC's) such as acetaldehyde, acetone,methyl ethyl ketone, butyraldehyde, crotonaldehyde, 2-ethylcrotonaldehyde, and 2-ethyl butyraldehyde and the like, and the aldolcondensation products thereof. The present invention may also lead toreduction of propionic acid.

The carbonyl impurities described above, such as acetaldehyde, may reactwith iodide catalyst promoters to form multi-carbon alkyl iodides, e.g.,ethyl iodide, propyl iodide, butyl iodide, pentyl iodide, hexyl iodide,and the like. It is desirable to remove multi-carbon alkyl iodides fromthe reaction product because even small amounts of these impurities inthe acetic acid product tend to poison the catalyst used in theproduction of vinyl acetate, a product commonly produced from aceticacid. Thus, the present invention may also lead to reduction or removalof multi-carbon alkyl iodides, in particular C₂₋₁₂ alkyl iodidecompounds. Accordingly, because many impurities originate withacetaldehyde, it is a primary objective to remove carbonyl impurities,notably acetaldehyde, from the process so as to reduce the multi-carbonalkyl iodide content.

Conventional techniques to remove such impurities include treating theacetic acid product streams with oxidizers, ozone, water, methanol,activated-carbon, amines, and the like. Such treatments may or may notbe combined with distillation of the acetic acid. The most typicalpurification treatment involves a series of distillations of the finalproduct. It is also known to remove carbonyl impurities from organicstreams by treating the organic streams with an amine compound such ashydroxylamine, which reacts with the carbonyl compounds to form oximes,followed by distillation to separate the purified organic product fromthe oxime reaction products. However, the additional treatment of thefinal product adds cost to the process, and distillation of the treatedacetic acid product can result in additional impurities being formed.

While it is possible to obtain acetic acid of relatively high purity,the acetic acid product formed by the low-water carbonylation processand purification treatment described above frequently remains somewhatdeficient with respect to the permanganate time due to the presence ofsmall proportions of residual impurities. Because a sufficientpermanganate time is an important commercial test, which the acidproduct may be required to meet to be suitable for many uses, thepresence of impurities that decrease permanganate time is objectionable.Moreover, it has not been economically or commercially feasible toremove minute quantities of these impurities from the acetic acid bydistillation because some of the impurities have boiling points close tothat of the acetic acid product or halogen-containing catalystpromoters, such as methyl iodide.

It has thus become important to identify economically viable methods ofremoving impurities elsewhere in the carbonylation process withoutcontaminating the final product or adding unnecessary costs. Forexample, a method for manufacturing high purity acetic acid by adjustingthe acetaldehyde concentration of the reaction solution below a certainamount, such as 1500 ppm, has been disclosed. It is stated that bymaintaining the acetaldehyde concentration below this threshold, it ispossible to suppress the formation of impurities such that one need onlydistill the crude acetic acid product to obtain high purity acetic acid.

The art has also disclosed that carbonyl impurities present in theacetic acid product streams generally concentrate in the overhead fromthe light ends column. Accordingly, the light ends column overhead hasbeen treated with an amine compound (such as hydroxylamine), whichreacts with the carbonyl compounds to form oxime derivatives that can beseparated from the remaining overhead by distillation, resulting in anacetic acid product with improved permanganate time.

Other processes have been described for producing high purity aceticacid in which it is stated that an acetaldehyde concentration of 400 ppmor less is maintained in the reactor by using distillation to removeacetaldehyde. Streams suggested for processing to remove acetaldehydeinclude a light phase containing primarily water, acetic acid and methylacetate; a heavy phase containing primarily methyl iodide, methylacetate and acetic acid; an overhead stream containing primarily methyliodide and methyl acetate; or a recirculating stream formed by combiningthe light and heavy phase.

It has been disclosed in commonly assigned U.S. Pat. Nos. 6,143,930 and6,339,171 that it is possible to significantly reduce the undesirableimpurities in the acetic acid product by performing a multi-stagepurification on the light ends column overhead. These patents disclose apurification process in which the light ends overhead is distilledtwice, in each case taking the acetaldehyde overhead and returning amethyl iodide rich residuum to the reactor. The acetaldehyde-richdistillate obtained after the two distillation steps is optionallyextracted with water to remove the majority of the acetaldehyde fordisposal, leaving a significantly lower acetaldehyde concentration inthe raffinate that is recycled to the reactor. U.S. Pat. Nos. 6,143,930and 6,339,171 are incorporated herein by reference in their entirety.

While the above-described processes have been successful in removingcarbonyl impurities from the carbonylation system and for the most partcontrolling acetaldehyde levels and permanganate time problems in thefinal acetic acid product, further improvements can still be made.Accordingly, there remains a need for alternative processes to improvethe efficiency of acetaldehyde removal. The present invention providesone such alternative solution.

SUMMARY OF THE INVENTION

This invention relates to a process for the production of acetic acidand, in particular, an improved process for the reduction and/or removalof permanganate reducing compounds and alkyl iodides formed by thecarbonylation of methanol in the presence of a Group VIII metalcarbonylation catalyst to produce acetic acid. More specifically, thisinvention relates to an improved process for reducing and/or removingpermanganate reducing compounds or their precursors from intermediatestreams during the formation of acetic acid by said carbonylationprocesses.

In one aspect, the present invention provides a process for thereduction and/or removal of permanganate reducing compounds (PRC's)formed in the carbonylation of a carbonylatable reactant to produce acarbonylation product comprising acetic acid. The process comprises thesteps of: (a) separating the carbonylation product to provide a vaporoverhead stream comprising acetic acid and a less volatile catalystphase; (b) distilling the vapor overhead stream to yield a purifiedacetic acid product and a low boiling overhead vapor stream comprisingmethyl iodide, water, acetic acid, methyl acetate, and at least one PRC;(c) condensing the low boiling overhead vapor stream and biphasicallyseparating it to form a condensed heavy liquid phase and a condensedlight liquid phase; (d) distilling the condensed light liquid phase in asingle distillation column to form a second vapor phase stream overheadand a higher boiling liquid phase residuum, wherein the second vaporphase stream is enriched with PRC's with respect to the condensed lightliquid phase; and (e) condensing the second vapor phase stream andextracting the condensed stream with water to obtain an aqueousacetaldehyde stream comprising PRC and a raffinate comprising methyliodide. In certain variations, the process can be operated with orwithout a sidestream comprising methyl acetate being taken from thedistillation column of step (d).

In another aspect, the present invention provides a process for thereduction and/or removal of permanganate reducing compounds (PRC's)formed in the carbonylation of a carbonylatable reactant to produce acarbonylation product comprising acetic acid, comprising the steps of:(a) separating the carbonylation product to provide a vapor overheadstream comprising acetic acid and a less volatile catalyst phase; (b)distilling the vapor overhead stream to yield a purified acetic acidproduct and a low boiling overhead vapor stream comprising methyliodide, water, acetic acid, methyl acetate, and at least one PRC; (c)condensing the low boiling overhead vapor stream and biphasicallyseparating it to form a condensed heavy liquid phase and a condensedlight liquid phase; (d) distilling the condensed light liquid phase in asingle distillation column to form a second vapor phase stream overheadand a higher boiling liquid phase residuum, wherein the second vaporphase stream is enriched with PRC's with respect to the condensed lightliquid phase and wherein the higher boiling liquid phase residuum isenriched with methyl acetate with respect to said second vapor phasestream overhead; and (e) condensing the second vapor phase stream andextracting the condensed stream with water to obtain an aqueousacetaldehyde stream comprising PRC and a raffinate comprising methyliodide. In certain variations, the process can be operated with orwithout a sidestream comprising methyl acetate being taken from thedistillation column of step (d)

In a third aspect, the present invention provides a process for thereduction and/or removal of permanganate reducing compounds (PRC's)formed in the carbonylation of a carbonylatable reactant to produce acarbonylation product comprising acetic acid, comprising the steps of:(a) separating the carbonylation product to provide a vapor overheadstream comprising acetic acid and a less volatile catalyst phase; (b)distilling the vapor overhead stream to yield a purified acetic acidproduct and a low boiling overhead vapor stream comprising methyliodide, water, acetic acid, methyl acetate, and at least one PRC; (c)condensing the low boiling overhead vapor stream and biphasicallyseparating it to form a condensed heavy liquid phase and a condensedlight liquid phase; (d) distilling the condensed light liquid phase in asingle distillation column to form a second vapor phase stream overheadand a higher boiling liquid phase residuum, wherein the second vaporphase stream is enriched with PRC's with respect to the condensed lightliquid phase; (e) removing a sidestream comprising methyl acetate fromthe distillation column of step (d), wherein the higher boiling liquidphase residuum and the sidestream are cumulatively enriched with methylacetate with respect to said second vapor phase stream; and (f)condensing the second vapor phase stream and extracting the condensedstream with water to obtain an aqueous acetaldehyde stream comprisingPRC and a raffinate comprising methyl iodide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates various embodiments of the present invention.

While the invention is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. Itshould be understood, however, that the invention is not intended to belimited to the particular forms disclosed. Rather, the invention isintended to cover all modifications, equivalents and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

This invention relates to a process for the production of acetic acidand, in particular, an improved process for the reduction and/or removalof permanganate reducing compounds formed by the carbonylation ofmethanol in the presence of a Group VIII metal carbonylation catalyst toproduce acetic acid. More specifically, this invention relates to animproved process for reducing and/or removing permanganate reducingcompounds or their precursors from intermediate streams during theformation of acetic acid by said carbonylation processes.

In particular, the present invention relates to a process in which acondensed light phase from a light ends column overhead is subjected toa single distillation to obtain an overhead that is subjected to anextraction to selectively reduce and/or remove PRC's from the process.Among other advantages, the present invention is able to reduce and/orremove PRC's from the process using a single distillation column andextraction combination as compared to previous process that utilizedmore than one distillation column with (or without) extraction to reduceand/or remove PRC's from a condensed light phase from a light endscolumn overhead. Additional advantages include, but are not limited to,lower energy usage and reduced equipment and associated costs.

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

The purification process of the present invention is useful in anyprocess used to carbonylate methanol (or another carbonylatablereactant, including, but not limited to, methyl acetate, methyl formateor dimethyl ether, or mixtures thereof) to acetic acid in the presenceof a Group VIII metal catalyst, such as rhodium, and ahalogen-containing catalyst promoter. A particularly useful process isthe low water rhodium-catalyzed carbonylation of methanol to acetic acidas exemplified in U.S. Pat. No. 5,001,259.

Generally, the rhodium component of the catalyst system is believed tobe present in the form of a coordination compound of rhodium with ahalogen component providing at least one of the ligands of suchcoordination compound. In addition to the coordination of rhodium andhalogen, it is also believed that carbon monoxide will coordinate withrhodium. The rhodium component of the catalyst system may be provided byintroducing into the reaction zone rhodium in the form of rhodium metal,rhodium salts such as the oxides, acetates, iodides, carbonates,hydroxides, chlorides, etc., or other compounds that result in theformation of a coordination compound of rhodium in the reactionenvironment.

The halogen-containing catalyst promoter of the catalyst system consistsof a halogen compound comprising an organic halide. Thus, alkyl, aryl,and substituted alkyl or aryl halides can be used. Preferably, thehalogen-containing catalyst promoter is present in the form of an alkylhalide. Even more preferably, the halogen-containing catalyst promoteris present in the form of an alkyl halide in which the alkyl radicalcorresponds to the alkyl radical of the feed alcohol, which is beingcarbonylated. Thus, in the carbonylation of methanol to acetic acid, thehalide promoter will include methyl halide, and more preferably methyliodide.

The liquid reaction medium employed may include any solvent compatiblewith the catalyst system and may include pure alcohols, or mixtures ofthe alcohol feedstock and/or the desired carboxylic acid and/or estersof these two compounds. A preferred solvent and liquid reaction mediumfor the low water carbonylation process contains the desired carboxylicacid product. Thus, in the carbonylation of methanol to acetic acid, apreferred solvent system contains acetic acid.

Water is contained in the reaction medium but desirably atconcentrations well below that which has heretofore been thoughtpractical for achieving sufficient reaction rates. It has previouslybeen taught, e.g., U.S. Pat. No. 3,769,329, that in rhodium-catalyzedcarbonylation reactions of the type set forth in this invention, theaddition of water exerts a beneficial effect upon the reaction rate.Thus, commercial operations are commonly run at water concentrations ofat least about 14 wt %. Accordingly, it has been quite unexpected thatreaction rates substantially equal to and above reaction rates obtainedwith such comparatively high levels of water concentration can beachieved with water concentrations below 14 wt % and as low as about 0.1wt %.

In accordance with the carbonylation process most useful to manufactureacetic acid according to the present invention, the desired reactionrates are obtained even at low water concentrations by maintaining inthe reaction medium an ester of the desired carboxylic acid and analcohol, desirably the alcohol used in the carbonylation, and anadditional iodide ion that is over and above the iodide ion that ispresent as hydrogen iodide. A desired ester is methyl acetate. Theadditional iodide ion is desirably an iodide salt, with lithium iodidebeing preferred. It has been found, e.g., U.S. Pat. No. 5,001,259, thatunder low water concentrations, methyl acetate and lithium iodide act asrate promoters only when relatively high concentrations of each of thesecomponents are present and that the promotion is higher when both ofthese components are present simultaneously. The concentration of iodideion maintained in the reaction medium of the preferred carbonylationreaction system is believed to be quite high as compared with whatlittle prior art there is dealing with the use of halide salts inreaction systems of this sort. The absolute concentration of iodide ioncontent is not a limitation on the usefulness of the present invention.

The carbonylation reaction of methanol to acetic acid product may becarried out by contacting the methanol feed with gaseous carbon monoxidebubbled through a acetic acid solvent reaction medium containing therhodium catalyst, methyl iodide promoter, methyl acetate, and additionalsoluble iodide salt, at conditions of temperature and pressure suitableto form the carbonylation product. It will be generally recognized thatit is the concentration of iodide ion in the catalyst system that isimportant and not the cation associated with the iodide, and that at agiven molar concentration of iodide the nature of the cation is not assignificant as the effect of the iodide concentration. Any metal iodidesalt, or any iodide salt of any organic cation, or quaternary cationsuch as a quaternary amine or phosphine or inorganic cation can bemaintained in the reaction medium provided that the salt is sufficientlysoluble in the reaction medium to provide the desired level of theiodide. When the iodide is a metal salt, preferably it is an iodide saltof a member of the group consisting of the metals of Group IA and GroupIIA of the periodic table as set forth in the “Handbook of Chemistry andPhysics” published by CRC Press, Cleveland, Ohio, 2002-03 (83rdedition). In particular, alkali metal iodides are useful, with lithiumiodide being particularly suitable. In the low water carbonylationprocess most useful in this invention, the additional iodide ion overand above the iodide ion present as hydrogen iodide is generally presentin the catalyst solution in amounts such that the total iodide ionconcentration is from about 2 to about 20 wt % and the methyl acetate isgenerally present in amounts of from about 0.5 to about 30 wt %, and themethyl iodide is generally present in amounts of from about 5 to about20 wt %. The rhodium catalyst is generally present in amounts of fromabout 200 to about 2000 parts per million (ppm).

Typical reaction temperatures for carbonylation will be from about 150to about 250° C., with the temperature range of about 180 to about 220°C. being a preferred range. The carbon monoxide partial pressure in thereactor can vary widely but is typically about 2 to about 30atmospheres, and preferably, about 3 to about 10 atmospheres. Because ofthe partial pressure of by-products and the vapor pressure of thecontained liquids, the total reactor pressure will range from about 15to about 40 atmospheres.

A typical reaction and acetic acid recovery system that is used for theiodide-promoted rhodium catalyzed carbonylation of methanol to aceticacid in accordance with the present invention is shown in FIG. 1 andincludes a liquid phase carbonylation reactor, flasher, and a methyliodide acetic acid light ends column (“light ends column”) 14. In theprocess, carbonylation product obtained in the reactor is provided tothe flasher where a volatile (“vapor”) overhead stream comprising aceticacid and a less volatile catalyst phase (catalyst-containing solution)are obtained. The volatile overhead stream comprising acetic acid isprovided by stream 26 to the light ends column 14 where distillationyields a purified acetic acid product that is removed via sidestream 17and an overhead distillate stream 28 (hereafter “low-boiling overheadvapor stream”). Acetic acid removed via sidestream 17 can be subjectedto further purification, such as to a drying column for selectiveseparation of acetic acid from water.

The reactor and flasher are not shown in FIG. 1. These are consideredstandard equipment now well known in the carbonylation process art. Thecarbonylation reactor is typically either a stirred vessel orbubble-column type within which the reacting liquid or slurry contentsare maintained automatically at a constant level. Into this reactor,there are continuously introduced fresh methanol, carbon monoxide,sufficient water as needed to maintain at least a finite concentrationof water in the reaction medium. Also introduced into the reactor is arecycled catalyst solution, such as from the flasher base, a recycledmethyl iodide phase, a recycled methyl acetate phase, and a recycledaqueous acetic acid phase. A recycled phase may contain one or more ofthe foregoing components.

Distillation systems are employed that provide means for recovering thecrude acetic acid and recycling catalyst solution, methyl iodide, methylacetate, and other system components within the process. In a typicalcarbonylation process, carbon monoxide is continuously introduced intothe carbonylation reactor, desirably below the agitator, which is usedto stir the contents. The gaseous feed is thoroughly dispersed throughthe reacting liquid by this stirring means. A gaseous purge stream isdesirably vented from the reactor to prevent buildup of gaseousby-products and to maintain a set carbon monoxide partial pressure at agiven total reactor pressure. The temperature of the reactor iscontrolled and the carbon monoxide feed is introduced at a ratesufficient to maintain the desired total reactor pressure.

Liquid product is drawn off from the carbonylation reactor at a ratesufficient to maintain a constant level therein and is introduced to theflasher. In the flasher, a catalyst-containing solution (catalyst phase)is withdrawn as a base stream (predominantly acetic acid containing therhodium and the iodide salt along with lesser quantities of methylacetate, methyl iodide, and water), while a vapor overhead streamcomprising acetic acid is withdrawn overhead. The vapor overhead streamcomprising acetic acid also contains methyl iodide, methyl acetate, andwater. Dissolved gases exiting the reactor and entering the flashercomprise a portion of the carbon monoxide and may also contain gaseousby-products such as methane, hydrogen, and carbon dioxide. Suchdissolved gases exit the flasher as part of the overhead stream. Theoverhead stream is directed to the light ends column 14 as stream 26.

It has been disclosed in U.S. Pat. Nos. 6,143,930 and 6,339,171 thatthere is generally a higher concentration of the PRC's and in particularacetaldehyde content in the low-boiling overhead vapor stream 28 exitingcolumn 14 than in the high-boiling residue stream exiting column 14.Thus, in accordance with the present invention, low-boiling overheadvapor stream 28, containing PRC's is subjected to additional processingto reduce and/or remove the amount of PRC's present. Low-boilingoverhead vapor stream 28, therefore, is condensed and directed to anoverhead receiver decanter 16. In addition to PRC's, low-boilingoverhead vapor stream 28 will typically contain methyl iodide, methylacetate, acetic acid, and water. Conditions are desirably maintained inthe process such that low-boiling overhead vapor stream 28, once indecanter 16, will separate into a light phase and a heavy phase.Generally, low-boiling overhead vapor stream 28 is chilled to atemperature sufficient to condense and separate the condensable methyliodide, methyl acetate, acetaldehyde and other carbonyl components, andwater into two phases. A portion of stream 28 may include noncondensablegases such as carbon dioxide, hydrogen, and the like that can be ventedas shown in stream 29 on FIG. 1.

The condensed light phase in decanter 16 will generally comprise water,acetic acid, and PRC's, as well as quantities of methyl iodide andmethyl acetate. The condensed heavy phase in decanter 16 will generallycomprise methyl iodide, methyl acetate, and PRC's.

The present invention may broadly be considered as an improved processfor distilling PRC's, primarily aldehydes such as acetaldehyde, from alow-boiling overhead vapor stream, particularly the condensed lightphase of a low-boiling overhead vapor stream 28 from a light endsdistillation column 14. In accordance with the present invention, acondensed light phase of a low-boiling overhead vapor stream 28 from alight ends distillation column 14 is distilled once and then subjectedto single- or multistage extraction to reduce and/or remove PRC's.

This process, such as that in the embodiments disclosed in FIG. 1, isdistinct from prior processes, such as that disclosed in U.S. Pat. No.6,339,171, including that illustrated in FIG. 1 of U.S. Pat. No.6,339,171.

In accordance with the present invention, disclosed in FIG. 1, thelow-boiling overhead vapor stream 28 contains methyl iodide, methylacetate, PRC's such as acetaldehyde and optionally other carbonylcomponents. The low-boiling overhead vapor stream 28 also contains waterand some quantity of acetic acid.

The low-boiling overhead vapor stream 28 is then condensed and separated(in vessel 16) to form a condensed heavy liquid phase containing thelarger proportion of methyl iodide, but also containing PRC's, and acondensed light liquid phase (taken of as stream 30), notably containingPRC's, water, and acetic acid but also generally containing somequantity of both methyl iodide and methyl acetate.

While either phase of the light ends overhead, i.e., low-boilingoverhead vapor stream 28, may be subsequently processed to remove thePRC's and primarily the acetaldehyde component of the stream, in thepresent invention, the PRC's are removed from the condensed light liquidphase 30.

Thus, the condensed heavy liquid phase in the decanter 16 can beconveniently recirculated, either directly or indirectly, to the reactor(not shown in FIG. 1). For example, a portion of this condensed heavyliquid phase can be recirculated to the reactor, with a slip stream,generally a small amount, e.g., 25 vol. %, preferably less than about 20vol. %, of the heavy liquid phase being directed to a carbonyl treatmentprocess. This slip stream of the heavy liquid phase may be treatedindividually or may be combined with the condensed light liquid phase 30for further distillation and extraction of carbonyl impurities inaccordance with the present invention.

In accordance with the present invention, condensed light liquid phase30 is directed to distillation column 18, which serves to form a secondvapor phase 36 enriched in PRC's, notably acetaldehyde, but alsocontaining methyl iodide due to the similar boiling points of methyliodide and acetaldehyde. Second vapor phase 36 is condensed and thenextracted with water to reduce and/or remove PRC's, notablyacetaldehyde. In a preferred embodiment a portion of the condensedstream 36 is provided as reflux to distillation column 18. This can beaccomplished, as shown in FIG. 1, by provided the condensed stream 36 toan overhead receiver 20, from which a portion of condensed stream 36 canbe provided to the extraction step (generally indicated as 70) by stream40 and another portion of condensed stream 36 can be provided as refluxto distillation column 18 by stream 42.

Acetaldehyde is extracted by water to obtain an aqueous acetaldehydestream 72, which will generally be treated as a waste. The raffinatefrom the extraction, notably containing methyl iodide is desirablyreturned to the carbonylation process by stream 74. In variousembodiments, aqueous acetaldehyde stream 72 can be stripped of aldehydefor treatment as waste with water being recirculated for use in theprocess, such as for the water used in extraction 70.

In the present invention, a primary concern is in the extraction stepthat separates acetaldehyde from methyl iodide. The efficiency of thisseparation is primarily affected by the relative solubility ofacetaldehyde and methyl iodide in water. While acetaldehyde is misciblein water, methyl iodide is not. However, the solubility of methyl iodidein water increases, with a concomitant loss of methyl iodide from theprocess system, with increasing levels of methyl acetate and/ormethanol. At high enough methyl acetate and/or methanol levels, phaseseparation of methyl iodide in the water extraction may not occur.Similarly, phase separation of methyl iodide in the water extraction maynot occur if acetic acid concentrations are sufficiently high. Thus, itis desirable that the distillate that is condensed and provided forextraction contain methanol and methyl acetate at a combinedconcentration of less than about 10 wt. %, more desirably less thanabout 5 wt. %, even more desirably less than about 2 wt. %, and evenmore desirably less than about 1.5 wt. %. It is desirable that thedistillate that is condensed and provided for extraction contain lessthan about 3 wt. % acetic acid, more desirably less than about 1 wt. %,and even more desirably less than about 0.5 wt. %. Particularly desiredwould be acetic acid concentrations approaching zero wt. %.

Thus, in the process of the present invention, a single distillation isconducted in distillation column 18 under conditions designed tocontrol, notably minimize, the quantities of methyl acetate and aceticacid in second vapor phase stream 36. Desirably, minimization ofquantities of methyl acetate and acetic acid in second vapor phasestream 36 is achieved while simultaneously maintaining higheracetaldehyde levels in second vapor phase stream 36 than in the residuumof distillation column 18. It is desirable that the residuum ofdistillation column 18 contain less than about 0.3 wt. % acetaldehyde,more desirably less than about 0.2 wt. %, and even more desirably lessthan about 0.1 wt. %. Particularly desired would be acetaldehydeconcentrations approaching zero wt. %.

In prior art processes, such as that illustrated in FIG. 1 of U.S. Pat.No. 6,339,171, two distillation steps are conducted in order to obtain afinal vapor distillate containing sufficiently low quantities of methylacetate, methanol, and acetic acid such that the distillate can besubjected to extraction with water to selectively separate acetaldehydefrom methyl iodide. In such prior art processes, water and acetic acidare preferentially separated as a residuum in the first column withmethyl acetate in the distillate. Subsequently, methyl acetate ispreferentially separated as a residuum in the second column. The priorart did not teach that methyl acetate, acetic acid, and water could beeffectively removed as residuum using a single distillation columnwithout having an undesirable concentration of acetaldehyde in theresiduum. Similarly, the prior art did not teach that acetaldehyde couldbe effectively concentrated in the distillate of a single column withoutobtaining an undesirable concentration of methyl acetate in thedistillate. As a result of their efforts, the present inventors havefound that such separations can be achieved using a single distillationstep, resulting in improved process efficiency.

Thus, in accordance with one embodiment of the present invention,illustrated in FIG. 1, low-boiling overhead vapor stream 28 is condensedin overhead receiver decanter 16 where it is biphasically separated toform a condensed heavy liquid phase and a light condensed liquid phase30. The light condensed liquid phase 30 is provided to distillationcolumn 18 via stream 30/32. In this and other embodiments of the presentinvention, a portion of stream 30 can be directed back to the light endscolumn 14 as reflux stream 34.

In distillation column 18, a second vapor phase stream 36 overhead and ahigher boiling liquid phase residuum stream 38 are formed. The secondvapor phase stream 36 overhead is enriched with PRC, notablyacetaldehyde, with respect to the light condensed liquid phase 30. Thesecond vapor phase stream 36 overhead is deficient with methyl acetate,methanol, and/or acetic acid (desirably all three) with respect to saidlight condensed liquid phase 30. The higher boiling liquid phaseresiduum stream 38 is enriched with methyl acetate, methanol, and/oracetic acid (desirably all three) with respect to said second vaporphase stream 36. Desirably, the second vapor phase stream 36 overhead isenriched with PRC, notably acetaldehyde, with respect to the higherboiling liquid phase residuum stream 38. The higher boiling liquid phaseresiduum stream 38 can be, and preferably is, retained in the process.

One of ordinary skill in the art having the benefit of this disclosurecan design and operate a distillation column to achieve the desiredresults of the present invention. Such efforts, although possiblytime-consuming and complex, would nevertheless be routine for one ofordinary skill in the art having the benefit of this disclosure.Accordingly, the practice of this invention is not necessarily limitedto specific characteristic of a particular distillation column or theoperation characteristics thereof, such as the total number of stages,the feed point, reflux ratio, feed temperature, reflux temperature,column temperature profile, and the like.

Further in accordance with this first embodiment of the presentinvention, second vapor phase stream 36 is extracted with water(generally indicated by 70) to remove and/or reduce PRC's, notablyacetaldehyde. Acetaldehyde is extracted by the water to obtain aqueousacetaldehyde stream 72, which is PRC-rich, and in particularacetaldehyde-rich. Aqueous acetaldehyde stream 72 will generally betreated as a waste, although in some embodiments acetaldehyde may bestripped, with the water being recirculated to the process. Theraffinate, notably containing methyl iodide is desirably returned to thecarbonylation process by stream 74. The efficiency of the extractionwill depend on such things as the number of extraction stages and thewater to feed ratio.

Extraction with water 70, in accordance with this first or otherembodiments of the present invention, can be either a singlestage ormultistage extraction and any equipment used to conduct such extractionscan be used in the practice of the present invention. Multistageextraction is preferred. For example, extraction 70 can be accomplishedby combining stream 40 with water and providing the combinationsuccessively to a mixer and then a separator. Multiple such combinationsof mixer and separator can be operated in series to obtain a multistageextraction. Optionally, and desirably, multistage extraction isaccomplished in a single vessel having a series of trays. The vessel maybe equipped with paddle(s) or other mechanisms for agitation to increaseextraction efficiency. In such a multistage extraction vessel, stream 40is desirably provided proximate to one end of the vessel with waterbeing provided proximate to the other end of the vessel or such otherlocation to obtain a countercurrent flow.

The mutual solubility between the two phases in the extraction canincrease with temperature. Accordingly, it is desirable that theextraction be conducted at a combination of temperature and pressuresuch that the extractor contents can be maintained in the liquid state.Moreover, it is desirable to minimize the temperatures to which stream40 is exposed to minimize the likelihood of polymerization andcondensation reactions involving acetaldehyde. Water used in theextraction 70 is desirably from an internal stream so as to maintainwater balance within the reaction system. Dimethyl ether (DME) can beintroduced to the extraction to improve the separation of methyl iodidein the extraction, i.e., to reduce the loss of methyl iodide into theaqueous acetaldehyde stream 72. The DME can be introduced to the processor formed in situ.

In accordance with a second embodiment of the present invention, alsoillustrated in FIG. 1, low-boiling overhead vapor stream 28 is condensedin decanter 16 where it is biphasically separated to form a condensedheavy liquid phase and a condensed light liquid phase 30. The condensedlight liquid phase 30 is provided to distillation column 18 via stream30/32. Again, in this and other embodiments of the present invention, aportion of stream 30 can be directed back to the light ends column 14 asreflux stream 34. In distillation column 18, a second vapor phase stream36 overhead and a higher boiling liquid phase residuum stream 38 areformed. A sidestream 80, comprising methyl acetate, is also taken.

The sidestream 80 allows the distillation column 18 to be operated underconditions desirable for obtaining a higher concentration ofacetaldehyde in second vapor phase stream 36 while providing a mechanismfor removing methyl acetate that might otherwise build up in the centerof distillation column 18 or be pushed into the second vapor phasestream 36 overhead. The sidestream 80, comprising methyl acetate, ispreferably retained in the process.

In this second embodiment, the second vapor phase stream 36 overhead isenriched with PRC, notably acetaldehyde, with respect to light condensedliquid phase 30. The second vapor phase stream 36 overhead is deficientwith methyl acetate, methanol, and/or acetic acid (desirably all three)with respect to light condensed liquid phase 30. The second vapor phasestream 36 overhead is deficient with methyl acetate, methanol, and/oracetic acid (desirably all three) with respect to said sidestream 80and, desirably, also with respect to the higher boiling liquid phaseresiduum stream 38. Desirably, the second vapor phase stream 36 overheadis enriched with PRC, notably acetaldehyde, with respect to both thesidestream 80 and the higher boiling liquid phase residuum stream 38.

Further in accordance with this second embodiment of the presentinvention, second vapor phase stream 36 is extracted with water(generally indicated by 70) to remove residual PRC's, notablyacetaldehyde. Extraction in accordance with this second embodiment isconducted in accordance with the extraction procedures disclosed for thefirst embodiment.

Operating in accordance with the first embodiment without a sidestream,the process has been found to achieve the following results respectingthe separation capabilities of distillation column 18:

Weight Weight Weight Percent in Percent in Percent in Component Stream30/32 Stream 36 Stream 38 Methyl iodide 1.5 74.5 <0.1 Methyl acetate 6.01.4 6.1 Methanol 4.0 0.2 4.1 Acetic acid 15 <0.1 15.3 Water 73 1.6 74.5Acetaldehyde 0.5 22.2 0.1

Operating in accordance with the second embodiment with a sidestream, itis expected that the following results respecting the separationcapabilities of distillation column 18 can be achieved:

Weight Weight Weight Weight Percent in Percent in Percent in Percent inComponent Stream 30/32 Stream 36 Stream 38 Stream 80 Methyl iodide 1.546.8 <0.1 28.7 Methyl acetate 4.0 0.4 1.7 60.4 Methanol 1.0 <0.1 1.0 0.5Acetic acid 15 <0.1 15.7 0.5 Water 78 0.8 81.6 7.4 Acetaldehyde 0.5 52<0.1 2.5

This inventive process has been found to reduce and/or remove PRC's andtheir precursors, multi-carbon alkyl iodide impurities, and propionicand higher carboxylic acids from the carbonylation process. It has alsobeen shown that acetaldehyde and its derivatives are reduced and/orremoved by sufficient amounts such that it is possible to keep theconcentration of propionic acid in the acetic acid product below about500 parts per million by weight, preferably below about 300 parts permillion, and most preferably below 250 parts per million.

In variations of the embodiments of the present invention, it isimportant to inhibit the formation of various aldehyde related polymersand condensation products in distillation column 18. Acetaldehydepolymerizes to form metaldehyde and paraldehyde. These polymersgenerally are low molecular weight, less than about 200. Highermolecular weight polymers of acetaldehyde can also form. These highermolecular weight polymers (molecular weight greater than about 1000) arebelieved to form during processing of the light phase and are viscousand thixotropic. Acetaldehyde can also undergo undesirable aldolcondensation reactions.

The formation of these impurities, i.e., metaldehyde and paraldehyde andhigher molecular weight polymers of acetaldehyde, can be suppressed byintroducing into distillation column 18 a flush stream containing atleast water and/or acetic acid.

While the invention has been described with reference to the preferredembodiments, obvious modifications and alterations are possible by thoseskilled in the related art having the benefits of this disclosure.Therefore, it is intended that the invention include all suchmodifications and alterations to the full extent that they come withinthe scope of the following claims or the equivalents thereof.

1. A process for the reduction and/or removal of permanganate reducingcompounds (PRC's) formed in the carbonylation of a carbonylatablereactant to produce a carbonylation product comprising acetic acid,comprising the steps of: (a) separating the carbonylation product toprovide a vapor overhead stream comprising acetic acid and a lessvolatile catalyst phase; (b) distilling the vapor overhead stream toyield a purified acetic acid product and a low boiling overhead vaporstream comprising methyl iodide, water, acetic acid, methyl acetate, andat least one PRC; (c) condensing the low boiling overhead vapor streamand biphasically separating it to form a condensed heavy liquid phaseand a condensed light liquid phase; (d) distilling the condensed lightliquid phase in a single distillation column to form a second vaporphase stream overhead and a higher boiling liquid phase residuum,wherein the second vapor phase stream is enriched with PRC's withrespect to the condensed light liquid phase; and (e) condensing thesecond vapor phase stream and extracting the condensed stream with waterto obtain an aqueous acetaldehyde stream comprising PRC and a raffinatecomprising methyl iodide.
 2. The process of claim 1, wherein the totalconcentration of methyl acetate and methanol in the second vapor streamis less than about 5 wt. %.
 3. The process of claim 1, wherein theconcentration of acetaldehyde in the higher boiling liquid phaseresiduum is less than about 0.3 wt %.
 4. The process of claim 1, whereinthe second vapor phase stream is extracted in the presence of dimethylether.
 5. The process of claim 1 wherein step (d) is performed withoutremoving a sidestream comprising methyl acetate from the distillationcolumn and wherein the higher boiling liquid phase residuum is enrichedwith methyl acetate with respect to said second vapor phase stream. 6.The process of claim 5, wherein the total concentration of methylacetate and methanol in the second vapor stream is less than about 5 wt.%.
 7. The process of claim 5, wherein the concentration of acetaldehydein the higher boiling liquid phase residuum is less than about 0.3 wt %.8. The process of claim 1, further comprising removing a sidestreamcomprising methyl acetate from the distillation column of step (d),wherein the higher boiling liquid phase residuum and the sidestream arecumulatively enriched with methyl acetate with respect to said secondvapor phase stream.
 9. The process of claim 8, wherein the totalconcentration of methyl acetate and methanol in the second vapor streamis less than about 5 wt. %.
 10. The process of claim 8, wherein theconcentration of acetaldehyde in the higher boiling liquid phaseresiduum is less than about 0.3 wt %.
 11. A process for the reductionand/or removal of permanganate reducing compounds (PRC's) formed in thecarbonylation of a carbonylatable reactant to produce a carbonylationproduct comprising acetic acid, comprising the steps of: (a) separatingthe carbonylation product to provide a vapor overhead stream comprisingacetic acid and a less volatile catalyst phase; (b) distilling the vaporoverhead stream to yield a purified acetic acid product and a lowboiling overhead vapor stream comprising methyl iodide, water, aceticacid, methyl acetate, and at least one PRC; (c) condensing the lowboiling overhead vapor stream and biphasically separating it to form acondensed heavy liquid phase and a condensed light liquid phase; (d)distilling the condensed light liquid phase in a single distillationcolumn to form a second vapor phase stream overhead and a higher boilingliquid phase residuum, wherein the second vapor phase stream is enrichedwith PRC's with respect to the condensed light liquid phase and whereinthe higher boiling liquid phase residuum is enriched with methyl acetatewith respect to said second vapor phase stream overhead; and (e)condensing the second vapor phase stream and extracting the condensedstream with water to obtain an aqueous acetaldehyde stream comprisingPRC and a raffinate comprising methyl iodide.
 12. The process of claim11, wherein step (d) is performed without removing a sidestreamcomprising methyl acetate from the distillation column.
 13. The processof claim 12, wherein the total concentration of methyl acetate andmethanol in the second vapor stream is less than about 5 wt. %.
 14. Theprocess of claim 12, wherein the concentration of acetaldehyde in thehigher boiling liquid phase residuum is less than about 0.3 wt %. 15.The process of claim 11, comprising an additional step of removing asidestream comprising methyl acetate from the distillation column ofstep (d), wherein the higher boiling liquid phase residuum and thesidestream are cumulatively enriched with methyl acetate with respect tothe second vapor phase stream.
 16. The process of claim 15, wherein thetotal concentration of methyl acetate and methanol in the second vaporstream is less than about 5 wt. %.
 17. The process of claim 15, whereinthe concentration of acetaldehyde in the higher boiling liquid phaseresiduum is less than about 0.3 wt %.
 18. A process for the reductionand/or removal of permanganate reducing compounds (PRC's) formed in thecarbonylation of a carbonylatable reactant to produce a carbonylationproduct comprising acetic acid, comprising the steps of: (a) separatingthe carbonylation product to provide a vapor overhead stream comprisingacetic acid and a less volatile catalyst phase; (b) distilling the vaporoverhead stream to yield a purified acetic acid product and a lowboiling overhead vapor stream comprising methyl iodide, water, aceticacid, methyl acetate, and at least one PRC; (c) condensing the lowboiling overhead vapor stream and biphasically separating it to form acondensed heavy liquid phase and a condensed light liquid phase; (d)distilling the condensed light liquid phase in a single distillationcolumn to form a second vapor phase stream overhead and a higher boilingliquid phase residuum, wherein the second vapor phase stream is enrichedwith PRC's with respect to the condensed light liquid phase; (e)removing a sidestream comprising methyl acetate from the distillationcolumn of step (d), wherein the higher boiling liquid phase residuum andthe sidestream are cumulatively enriched with methyl acetate withrespect to said second vapor phase stream; and (f) condensing the secondvapor phase stream and extracting the condensed stream with water toobtain an aqueous acetaldehyde stream comprising PRC and a raffinatecomprising methyl iodide.
 19. The process of claim 18, wherein the totalconcentration of methyl acetate and methanol in the second vapor streamis less than about 5 wt. %.
 20. The process of claim 18, wherein theconcentration of acetaldehyde in the higher boiling liquid phaseresiduum is less than about 0.3 wt %.